Since the alloys capable of absorbing and desorbing hydrogen (which are abbreviated as "hydrogen absorbing alloys" hereinafter) were discovered, they have been applied to not only a hydrogen absorbing means but also a battery and the like. In particular, alkaline secondary batteries utilizing such alloys were already used practically, and the hydrogen absorbing alloys used therein have undergone successive improvements with the intention of heightening the battery capacity and lengthening the battery life.
More specifically, in the hydrogen absorbing alloy having a composition of LaNi.sub.5 and a CaCu.sub.5 type crystal structure, which was studied at first, it has been tried to partially replace the La by other rare earth elements, such as Ce, Pr and Nd, and the Ni by other metal elements, such as Al, Co and Mn, thereby enabling the secondary battery to have a higher capacity and a longer life.
However, such a replacement in the hydrogen absorbing alloy caused a drop in the initial activity of a secondary battery when the resultant alloy was used for a negative electrode of the battery although it was successful in heightening the battery capacity and lengthening the battery life.
The initial activity of a secondary battery is, in general, expressed in terms of the number of charge-discharge cycles repeated until the battery reaches the maximum capacity, so that the initial activity is considered to be higher the smaller such a number is. Usually, the initial activity is evaluated by the capacity in the first cycle and the number of charge-discharge cycles repeated until the battery reaches the maximum capacity.
When a secondary battery having a low initial activity is sealed hermetically, the balance between positive and negative electrodes is lost to decrease the battery capacity and the battery life.
In order to solve the aforementioned problem, it has so far been carried out to treat hydrogen absorbing alloys with acids or alkalis. However, these treatments have defects that not only the alloy is subject to surface oxidation during treatment but also the treated alloy is liable to oxidation upon drying and storage, so that they are insufficient in the ease of the handling and cannot ensure sufficient stability in the treated alloy.